in my opinion, why is bitcoin never a global currency, because every country already has its own currency, if you want to put bitcoin in it, of course there is a process that involves the entire country. If this is the case I find it difficult to make bitcoin a global currency ... !!
Plus one on the point you stated about every country having their own currency.
Probably when there is a world single power, then there will be a one single currency and Bitcoin might be part of it, aside that, Bitcoin can only work in line with every country currency to be functional. That is if it is acceptable in that country.